1. Field of the Invention
The present invention relates to a method for forming a foamed polypropylene resin sheet and a foamed formed article obtained by the same. More specifically, it relates to a method for forming a foamed polypropylene resin sheet in which draw-down and waving are not caused in heating the foamed sheet and in which inferior appearance such as partial thickness and wrinkles is not observed on the product, and a foamed formed article having an excellent shape obtained by the above method.
2. Description of the Related Art
A foamed article formed from a foamed polypropylene resin sheet is light-weight and excellent in a stiffness, a heat insulating property and a heat resistance, so that it is used in diversified fields. Such a foamed formed article is obtained by forming a foamed resin sheet, which has been softened by heating, by a pressure forming method or a vacuum forming method using a forming mold. However, a polypropylene resin has a low melt tension, and therefore there have so far been the problems that so-called draw-down and waving are liable to be caused on the foamed sheet in heating and that the resulting product is liable to be inferior in appearance due to partial thickness and wrinkles. Such problems are noticeable when a foamed sheet is large-sized in an area or the heating temperature is elevated in order to shorten the forming cycle, thus preventing the improvement in productivity.
An object of the present invention is to provide a method for forming a foamed polypropylene resin sheet in which draw-down and waving are not caused in heating the foamed sheet and in which inferior appearance such as partial thickness and wrinkles is not produced on the product, and to provide a foamed formed article having an excellent shape obtained by the method, thereby solving the problems described above.
Intensive investigations repeated by the present inventors in order to solve the problems described above have resulted in finding that the object described above can be achieved by using a specific foamed polypropylene resin sheet and heating it while supported on a tabular sheet support, and thus they have come to complete the present invention based on this knowledge.
The present invention comprises the following structures.
(1) A method for forming a foamed polypropylene resin sheet comprising a heating step for heating and softening the foamed polypropylene resin sheet and a forming step for bringing the heated foamed polypropylene resin sheet into contact with a forming mold to form it into a prescribed shape, wherein the foamed polypropylene resin sheet has an expansion ratio of 1.1 to 6 times, an open cell rate of 70% or less and a sheet thickness of 0.5 to 5 mm; and the foamed polypropylene resin sheet is heated in the heating step in the state that it is supported on a tabular sheet support.
(2) The method for forming a foamed polypropylene resin sheet as described in the above item (1), wherein the sheet support is heated to 160-210xc2x0 C. in the heating step.
(3) The method for forming a foamed polypropylene resin sheet as described in the above item (1) or (2), wherein a differential pressure of 0.05 to 1 MPa is applied between the upper face and the under face of the foamed polypropylene resin sheet in the forming step.
(4) The method for forming a foamed polypropylene resin sheet as described in any of the above items (1) to (3), wherein a polypropylene resin constituting the foamed polypropylene resin sheet satisfies the following equation:
log MT greater than 4.24xc3x97log [xcex7]xe2x88x921.2
wherein MT represents a melt tension (unit: cN) at 230xc2x0 C., and [xcex7] represents an intrinsic viscosity (unit: dl/g) measured in a tetralin of 135xc2x0 C.
(5) The method for forming a foamed polypropylene resin sheet as described in any of the above items (1) to (4), wherein the polypropylene resin constituting the foamed polypropylene resin sheet is an olefin polymer composition (A) shown below or a polypropylene resin composition comprising 10 to 99% by weight of the olefin polymer composition (A) and 90 to 1% by weight of a polypropylene resin (B): the olefin polymer composition (A) comprising:
(a) 0.01 to 5.0 parts by weight of a high molecular weight polyethylene having an ethylene polymer unit of 50% by weight or more, wherein the intrinsic viscosity [xcex7E] measured in a tetralin of 135xc2x0 C. falls in the range of 15 to 100 dl/g, and
(b) 100 parts by weight of a propylene homopolymer or a propylene-olefin copolymer having a propylene polymer unit of 50% by weight or more, wherein the intrinsic viscosity [xcex7p] measured in a tetralin of 135xc2x0 C. falls in the range of 0.2 to 10 dl/g, and
the polypropylene resin (B): a propylene homopolymer or a propylene/xcex1-olefin copolymer comprising propylene as a main component.
(6) A foamed polypropylene resin formed article formed by the method as described in any of the above items (1) to (5).